The technique using an OTDR (Optical Time Domain Reflectometer) is known as an optical fiber sensing technique by which Rayleigh backscattered light (referred to hereinbelow simply as “Rayleigh scattered light”) produced by injection of probe light into an optical fiber is used to measure line characteristics of the optical fibers (loss or position of breaks in the optical fiber) or the distribution of strains in the longitudinal direction of the optical fiber.
For example, a method described in Japanese Patent Application Publication No. H9-236513 is known as a method for measuring a line characteristic in the OTDR. With this method, probe light is inputted (incident) into an optical fiber and the line characteristic is measured on the basis of Rayleigh scattered light produced inside the fiber by such an input.
More specifically, an optical pulse is inputted (incident) as the probe light from one end (input end) of the optical fiber. The Rayleigh scattered light that has been produced in various regions in the longitudinal direction of the optical fiber by the input of the optical pulse and returned to the input end is measured. The line characteristic of the optical fiber is measured from the intensity of the measured Rayleigh scattered light and the position where the Rayleigh scattered light has been produced. In this case, the position in the longitudinal direction of the optical fiber where the Rayleigh scattering has occurred is specified on the basis of the reciprocation time required for the optical pulse inputted from the input time to be reflected inside the optical bier and return to the input end.
Further, for example, a method for using the frequency shift of Rayleigh scattered light produced in a region where strains have occurred in an optical fiber is known as a method for measuring the distribution of strains in the longitudinal direction of the optical fiber in the OTDR.
With this method, an optical pulse is inputted (incident) from the input end of the optical fiber. The Rayleigh scattered light that has been produced in various regions in the longitudinal direction of the optical fiber by the input of the optical pulse and has returned to the input end is measured. Where a pressure is applied to the optical fiber and strains are produced inside the optical bier, there is a shift in frequency of the Rayleigh scattered light produced in the region where the strains have appeared. As a result, the phase of the measured Rayleigh scattered light changes with respect to that of the Rayleigh scattered light produced in the optical fiber in the initial state (state in which the pressure has not been applied). This phase variation makes it possible to detect the pressure applied to the optical fiber. In this case, the phase variation can be obtained with good accuracy by inputting the optical pulse a plurality of times and finding the average of the Rayleigh scattered light in the various regions.
Thus, in the OTDR, the phase variation of Rayleigh scattered light in various regions in the longitudinal direction of the optical fibers can be detected, and strains (pressure applied to the optical fiber) in the regions in the longitudinal direction of the optical fiber can be detected with high sensitivity and high accuracy on the basis of the phase variation.
The method using the abovementioned phase variation of Rayleigh scattered light enables high-sensitivity and high-accuracy detection of strains produced in various regions in the longitudinal direction of the optical fiber. Therefore, a sound wave that has struck (reached) the regions of the optical fiber apparently can be detected by using such a method.
More specifically, where a certain sound wave propagates in a gas, liquid, or solid medium and reaches, that is, strikes, an optical fiber, tiny strains appear in the optical fiber. The strains depend on the frequency or amplitude of the sound wave that has struck the optical fiber. Accordingly, the strains produced in the regions of the optical fiber can be detected by the method using the abovementioned phase variation, and by analyzing the strains, it is possible to detect the sound wave (frequency or amplitude) and specify the position of the sound wave emission source.
In such a method, the resolution in the longitudinal direction (longitudinal resolution) when the sound wave is detected in the regions in the longitudinal direction of the optical fiber is determined by the pulse width of the probe light (optical pulse) inputted to the optical fiber. For example, when optical waves that have struck two points spaced in the longitudinal direction of the optical fiber are detected, where the spacing of the points is less than the pulse width of the probe light, it is impossible to determine which of the sound waves that have struck the two points has caused the derived phase variation of the Rayleigh scattered light. Therefore, the pulse width should be reduced to realize a high longitudinal resolution.
However, where the pulse width of the probe light is reduced, the energy of optical pulses decreases. As a result, the signal strength of the scattered light that is scattered in the regions of the optical fiber and returns to the input end decreases.
Further, in the method using the abovementioned phase variation, strains (strains of the optical fiber), which do not fluctuate over a short period of time, are detected. Therefore, the phase variation of the Rayleigh scattered light in each region is determined with good accuracy by measuring multiple times the Rayleigh scattered light produced in each region of the optical fiber and using the average thereof. However, since the strains in an optical fiber caused by a sound wave striking thereupon change over a very short period of time, the method of measuring multiple times the Rayleigh scattered light produced in the regions and using the average thereof cannot be used.
Thus, a sound wave is very difficult to detect with good accuracy by the method using the abovementioned phase variation.